Problem: Luis starts counting at 45, and he counts by threes. If 45 is the 1st number that Luis counts. what is the 15th number that he counts?
Answer: What is the first number that he counts? $45$ What is the second number that he counts? $\begin{align*}&45 + 3 \\ &= 48\end{align*}$ What is the third number that he counts? $\begin{align*}&45 + 3 + 3 \\ &= 45 + (2 \times 3) \\ &= 51\end{align*}$ What is the 15th number that he counts? $\begin{align*}&45 + (14\times3) \\ &= 45 + 42 \\ &= 87\end{align*}$